1. Field of the Invention
The present invention relates to the technical field of automatic gain control (AGC) and, more particularly, to a double-loop automatic gain controlling system based on hysteresis switching with a stably changing total gain.
2. Description of Related Art
FIG. 1 is a block diagram of automatic gain control in the prior art. An antenna 102 receives wireless communication signals. A coordinator 100 has a band-pass filter 104 for selecting a broadband spectrum including signals. A low noise amplifier 106 magnifies the broadband spectrum signals selected by the band-pass filter 104 according to a fixed magnification. A variable gain amplifier 108 magnifies the output signals of the low noise amplifier 106 according to a control signal VRF. A down converter 110 changes the frequency of the output signals of the variable gain amplifier 108 from the radio frequency to the intermediate frequency. A band-pass filter 112 executes narrowband filtering to filter and thus generate a narrowband signal. A variable gain amplifier 114 magnifies the output signals of the band-pass filter 112 according to a control signal VIF. An A/D converter 116 changes the output signals of the variable gain amplifier 114 to digital type. An automatic gain control device 120 outputs signals according to the A/D converter 116 and a power monitoring device 122 to generate the control signal VRF of the variable gain amplifier 108 and the control signal VIF of the variable gain amplifier 114.
For example, in the prior art, the output voltage VZ of the variable gain amplifier 114 is 110 dB μV approximately, and its corresponding voltage is 400-500 mV approximately, which corresponds to the range of input voltage of the A/D converter 116. If the input voltage Vx of the coordinator 100 is 60 dB μV approximately, which is the strength of the RF signal, and the band-pass filter 112 has a gain loss of −20 dB, it could be calculated that the sum of the gain of the coordinator 100 and the variable gain amplifier 114 is 110−(60−20)=70 dB μV, wherein the gain of the coordinator 114 includes the gain of the low noise amplifier 106 and the gain of the variable gain amplifier 108. However, since many reasons, such as the channel noise and the channel variability, may cause the input voltage Vx of the coordinator 100 to drift around 60 dB μV, the automatic gain control device 120 should adjust the control signals VRF and VIF.
FIG. 2 is a schematic diagram of operation of gain adjustment of automatic gain control in the prior art, which is divided into area I and area II according to the level of input voltage, namely, the input voltage Vx of the coordinator 100. In the area I, the control signal VRF is used to fix the gain of the variable gain amplifier 108 to a predetermined maximum gain GRFmax, and the control signal VIF is used to adjust the gain of the variable gain amplifier 114. In the area II, the control signal VIF is used to fix the gain of the variable gain amplifier 114 to a predetermined minimum gain GIFmin, and the control signal VRF is used to adjust the gain of the variable gain amplifier 108. The predetermined maximum gain GRFmax of the variable gain amplifier 108 is not the real maximum gain RFgain_max, and for performance and linear magnification, generally the predetermined maximum gain GRFmax is designed as slightly less than the maximum gain RFgain_max. For the same reason, the predetermined minimum gain GIFmin of the variable gain amplifier 114 is not the minimum gain IFgain_min, and generally the predetermined minimum gain GIFmin is designed larger than the minimum gain IFgain_min slightly.
As shown in FIG. 2, when the level of the input voltage Vx of the coordinator 100 is 60 dB μV, the variable gain amplifier 108 and the low noise amplifier 106 provide gain of 40 dB for GRFmax, and the variable gain amplifier 114 provides gain of 30 dB for GIFmin. When the level of the input voltage Vx of the coordinator 100 is 70 dB μV, the variable gain amplifier 114 provides fixed gain of 30 dB for GIFmin, and the variable gain amplifier 108 adjusts its gain according to the control voltage VRF outputted by the automatic gain control device 120, and the voltage VZ is 110 dB μV approximately, which is neither too large to exceed the range of input voltage of the A/D converter 116 nor too small to prevent the A/D converter 116 from proceeding conversion.
When the level of the input voltage Vx of the coordinator 100 is 50 dB μV, the variable gain amplifier 114 provides fixed gain of 40 dB for GRFmax, the variable gain amplifier 114 adjusts its gain according to the control voltage VIF outputted by the automatic gain control device 120, and the voltage VZ is 110 dB μV approximately.
However, when the level of the voltage Vx is approximately 60 dB μV, the whole automatic gain control system frequently switches between the area I and the area II. In the case, it not only easily generates low frequency noise due to switching, resulting in negatively affecting the gain adjustment of the automatic gain control system, but also easily makes the system instability.
Therefore, it is desirable to provide an improved automatic gain control system to mitigate and/or obviate the aforementioned problems.